1. Field of the Invention
The present invention is directed to an improvement of a valve mechanism in a slide type exhaust brake system, connected to the middle of an air release pipe, for imparting braking effects to a vehicle by blockading an exhaust passageway for an exhaust gas running through the air release pipe on the occasion of a sharp descent, typically in a large-sized vehicle such as a truck or a bus.
2. Description of the Prior Art
FIG. 7 illustrates a structure of a valve mechanism unit in this type of exhaust brake system. The valve mechanism consists of a tabular valve member (23) and an auxiliary valve member (27). The tabular valve member incorporated into a reciprocatory operation passageway of a housing of the body is formed as a single unit and includes an exhaust pressure adjusting hole (24) formed in a bottom wall surface thereof. The auxiliary valve member (27) is composed of a block member (25) fixed to a portion vicinal to the tip of a piston rod (26) and a liner unit (28) which is buffer-fitted to a bottom wall surface of a block member and slides on the surface of the valve member (23). When opening and closing the exhaust passageway, the tip of the piston rod (26) and a side wall surface of the block member (25) impinge upon and engage with the valve member (23) to provide up-and-down movements. Note that the numeral (29) represents an exhaust gas escape hole having a small diameter for preventing an excessive load on the machine body in a state where the exhaust passageway is closed.
The conventional valve mechanism is, however, attended with the following problems. Speaking of a structure of the auxiliary valve member (27), there exists a constraint to a size of the exhaust pressure adjusting hole (24) because of the slide surface of the valve member (23) in the limited space, and hence the exhaust pressure adjusting hole is not allowed to have a large diameter. As a result, a sufficient drop in exhaust pressure can not yet be attained. Besides, there is created abrasion or a "play" in the mutual impinging portion due to the engaging structure based on the impingement thereof when opening and closing the exhaust passageway. A life span of the product is reduced because of highly frequent repetition of opening/closing operations when being used, and hence the valve mechanism unit is short of durability.